The present invention generally relates to a temperature control apparatus and more particularly, to a temperature control arrangement for a heat roller which is used for fixing toner onto a copy material such as copy paper through heating, for example, in the electrophotographic copying apparatus. More specifically, the present invention relates to a temperature control arrangement of the above described type employing an infrared sensor unit which includes a pyroelectric infrared sensor, a chopper mechanism and a reference temperature measuring heat sensitive element.
Commonly, in an electrophotographic copying apparatus, for achieving a sufficient fixing performance by a fixing device employing a heat roller, it is necessary to effect control for maintaining the surface temperature (i.e.--fixing the temperature) of the heat roller at a predetermined value, since a faulty fixing may result if the fixing temperature is too low, while inconveniences such as the so-called offset toner phenomenon, etc., take place on the heat roller, if the fixing temperature is excessively high.
Conventionally, for the temperature control as described above, it has been a general practice to provide a heat sensitive element such as a thermistor or the like, directly contacting the surface of the heat roller (i.e.--a contact type detection) or confronting said surface through a very small space (i.e.--a non-contact type detection) so as to subject a heater element for said heat roller to an on/off control based on the temperature value detected by the heat sensitive element.
However, in the control method employing the heat sensitive element as described above, although the contact type detection has an advantage in that ripple or variation of the control temperature may be reduced owing to the instantaneous detection of the temperature of the heat roller, with a simultaneous suppression of an overshoot at the rising of the temperature of said heat roller, there are inconveniences in that the temperature detecting efficiency is undesirably lowered, with a reduction of response due to the soiling of the detecting portion caused by toner adhering to the surface of the heat roller, paper dust, oil, etc., and in that the surface of the heat roller tends to be damaged by the heat sensitive element held in contact therewith.
On the contrary, the non-contact type detection has advantages in that it is free from soiling at the detecting portion and damages to the heat roller surface, but there are shortcomings in that the heat sensitive element is readily affected by atmospheric air, with a poor thermal response due to the presence of an air layer between the heat sensitive element and the surface of the heat roller, thus making it difficult to effect an accurate temperature detection, while the ripple or variation in the controlled temperature tends to be large, resulting in the formation of fixing irregularities and faulty fixing, and thus, a high accuracy is required for the installation of the heat sensitive element.
In other words, under the present situation, the contact type detection and non-contact type detection respectively have merits and demerits as described so far, and therefore, a temperature detection system simultaneously having the merits of both detection types, with the elimination of the demerits thereof, i.e.--more specifically, a temperature detection system of a non-contact type which is free from the influence of the atmosphere has been needed.
Meanwhile, in a conventional fixing device, copy paper jamming arising from copy paper wound around the heat roller within the fixing device, etc. is arranged to be detected by a microswitch or a photo-interrupter or photo-sensor provided in the fixing device, but the former arrangement with the microswitch has a disadvantage caused by poor contact at contact portions thereof due to the vaporized fixing oil, while the latter arrangement employing the photo-interrupter has a problem with respect to its heat resistance.